Electric vehicles include battery packs which supply the electricity for their motors. These battery packs require frequent charging. The longest range which such vehicles have achieved with available battery packs is approximately 80 miles. One method however of recharging batteries is through stations with large, stationary chargers. This method ties vehicle owners to a relatively small geographic area near an available charging station. The better solution is to utilize an onboard charger so that the vehicle's battery pack can be charged wherever there is an electric outlet.
Onboard battery chargers require solutions to specific problems before they achieve practicality. The first problem is safety. Unless the battery charger isolates the power source from the battery pack, there is an unacceptable risk of fatal electric shock during charging. Chargers which do not include such isolation cannot be integrated into vehicles or utilized for any other purposes by the public.
The engineering solution must result in sufficient isolation for safety, while providing adequate charging power to charge vehicle battery packs quickly and at the same time keep charger size and weight to a minimum. The ratio of charging power to charger weight is known as "power density."
The second problem is correction for power factor and total harmonic distortion (THD). Power factor must be corrected to prevent large amounts of power which do not register on electric meters from being converted into second third, fifth and higher order haromonics which reflect back into the power grid causing large amounts of THD. See PG&E, The Impact of Electric Vehicle Battery Charging on the PG&E Distribution System, 1994.
Third, sixth, ninth, etc. harmonics heat neutral conductors in wiring and electrical panels, and the primary windings of utility pole or vault transformers and, over time, damage this costly equipment. Second, fifth, eighth etc. harmonics cause counter-rotating magnetic fields in electric motors so they draw more power to do less work, again leading eventually to costly equipment failures.
The power factor/THD problem therefore is a major, emerging priority for utility companies. The International Electrical and Electronic Engineers (IEEE) are currently discussing standards which will be mandated for all equipment which uses the power grid. See IEEE Proposed Standard 519. With thousands of electric vehicles anticipated by the end of this decade, imposition of this standard will require substantial improvement in power factor correction and THD for electric vehicle battery chargers.
A third problem is that constant current control schemes lead to rising power throughput, especially during the initial bulk charge step, during which most of the total charge energy is returned to the batteries. As a result power throughput reaches 100% of the charger's capability only in the last moment before bulk charge termination, and charging time is much longer than with constant power throughput as achieved in this invention.
The optimum number of charging steps varies with each battery management scheme chosen by each manufacturer. Usually these consist of several bulk charge steps followed by several equalization steps. The problem that must be addressed is to provide a sufficient number of programmable steps with programmable power levels to satisfy the charge algorithms of each battery manufacturer.
A fourth problem is the charge termination criterion used by a charger, that is, how a charger decides when to terminate each step of the charging process, and when to start each step. Most chargers use voltage leveling detection. This method is not as precise and is therefore inferior to "temperature compensated absolute voltage threshold detection." Absolute voltage threshold detection uses a temperature compensated absolute voltage reference so that the correct voltage threshold can be precisely established for charge termination. This is very important because the correct voltage to which a battery should be charged varies with the temperature of the battery.
A fifth problem is compatibility of the charger with the different voltages required for specific battery packs. Most chargers are designed to charge at a specific voltage level and cannot be used for charging when a battery pack with a different voltage level is installed.
Sixth and finally, electric vehicles require converters so that their high-voltage battery pack can be utilized to power the peripheral electrical systems of the vehicle, such as the radio, the cooling system and the windows. Vehicle manufacturers currently incorporate a separate DC to DC converter into their vehicles which adds an increment of cost to the vehicle's manufacture.